1. Field of the Invention
The invention is related to an arrangement coupled to a transducer which converts an electric signal to an acoustic or a mechanic signal for protecting the transducer against destruction caused by high amplitudes in transducer variables (temperature, displacement, force, stress, . . . ). The arrangement is connected to the electric transducer input and attenuates the electric signal supplied to the transducer in case of overload.
2. Description of the Prior Art
Transducers converting an electric signal into an acoustic or a mechanic signal (loudspeakers, headphones and actuators) can be endangered to malfunction or permanent destruction when a electric or mechanic variable in the transducer exceeds an allowed limit value. For example, the displacement of the voice coil of an electrodynamic transducer is limited by the geometry of the suspension and the motor structure.
Overloading the transducer can be prevented by operating the transducer with an amplifier supplying a maximal output power lower than the power handling capacity of the transducer. Input signals with high amplitude will always be limited by the amplifier and will not endanger the transducer. However, unpleasant distortions are generated if the amplifier is limiting.
Protecting the transducer by amplifier limiting is unacceptable in professional sound enhancement and initialized the development of special protection systems as disclosed in U.S. patent application Ser. No. 4490770 by H. R. Phillimore entitled OVERLOAD PROTECTION OF LOUDSPEAKERS, U.S. patent application Ser. No. 4330686 by R. Stephen entitled LOUDSPEAKER SYSTEMS, U.S. patent application Ser. No. 4301330 by T. Bruce entitled LOUDSPEAKER PROTECTION CIRCUIT, U.S. patent application Ser. No. 4296278 by S. B. Cullison entitled LOUDSPEAKER OVERLOAD PROTECTION CIRCUIT and U.S. patent application Ser. No. 3890465 by Y. Kaizu entitled CIRCUIT ARRANGEMENT FOR PROTECTION OF A SPEAKER SYSTEM. These systems will successfully protect the transducer against thermal overload related to the electric power supplied to the transducer but fail in the protection of the transducer against mechanical destruction caused by high amplitudes of mechanical variables.
Another protection circuit disclosed in U.S. patent application Ser. No. 4583245 by W. Gelow entitled SPEAKER SYSTEM PROTECTION CIRCUIT uses filters with controllable cutoff frequency for "altering the frequency range or speaker driving signal so as to remove the overload condition". A similar protection system described in The Mirror filter--a New Basis for Reducing Nonlinear Distortion Reduction and Equalizing Response in Woofer Systems by W. Klippel in J. Audio Eng. Soc. 32 (9), pp. 675-691, (1992) prevents clipping and mechanical damage caused by high voice coil displacement of an electrodynamic loudspeaker. The Mirror filter contains a linear reference filter with a transfer function H.sub.x (S) adjusted to the loudspeaker and provides an output signal x(t) related to the displacement of the voice coil. This reference filter is useful for monitoring the loudspeaker's displacement without a sensor and dispenses from additional sensing lines for feeding back the displacement signal into the protection circuit. The protection system contains a high-pass filter with controllable cutoff frequency. The high-pass filter is connected to the input of the mirror filter and provides a high-pass filtered input signal for the loudspeaker. For small voice coil excursion the protection system is not activated, and the filter cutoff frequency is at the low end of the audio range, about 20 Hz. If the voice coil displacement reaches the prescribed safe limit, the cutoff frequency of the high-pass filter is shifted upward, and the low frequency components are sufficiently removed from the system so that the displacement does not exceed the critical value. Thus distortion generation and the risk of destruction can be reduced by linear pre-filtering.
However, the protection systems of prior art has some disadvantages in practical implementation and in the performance with a real audio signal (music, speech).
Two filters have to implemented: One used as the reference filter for generating a signal related to the monitored transducer variable and another filter with controllable cut-off frequency for attenuating the electric input signal. The realization of the controllable high-pass filter is expensive because a complex circuit is required to fulfill the high performance requirements in the pass-band.
An important criteria of the performance is the reaction time which should be as short as possible to cope with steep attack slopes of transient input signals (e.g. signals from a bass drum). However, a controllable high-pass filter used as the attenuation element increases the reaction time of the protection system. If the protection system is not activated and the cut-off frequency is at the low end of the audio band (usually 20 Hz) the high-pass filter has a long impulse response. Supplying a transient input signal with high amplitudes, the cut-off frequency can not be increased in time to prevent an excess of the displacement over the allowed limit.
A short reaction time can be achieved by using an amplifier with controllable gain (VCA) instead of a high-pass filter with controllable cut-off frequency. However, the attenuation of all spectral component becomes audible and is superfluous if the overload is caused by a part of the spectrum.
A protection system having a transfer response under overload condition, which is directly related to the spectrum of the monitored signal, is not known in prior art.
Thus, there is a need for a protection circuit for loudspeakers which can provide an improved protection of the loudspeaker against electrical and mechanical overload caused by an arbitrary electric signal u(t), such as music, speech, secondary sound in active noise control.
A protection circuit is required which has a very short reaction time for coping with transient signals of high amplitude and for attenuating the electric signal at the transducer input in time.
Another object of the invention is to provide protection of the loudspeaker while causing a minimal change of the input signal. Thus a minimal amount of linear and nonlinear distortions are generated by the protection circuit. A protection circuit is required that attenuates the spectral components of the electric signal causing the overload of the transducer primarily.
A final purpose is to provide a reliable protection which can be realized with fewer elements and less complexity than prior art and implemented in a digital signal processing system at low costs.